1. Field of the Invention
This invention relates to new modified gellan gum compositions that show particular effects as well as to a process for the modification of gellan. The invention also relates to a process for the production of a gellan gum-containing product having a food or non-food application and to a gum-containing product which comprises such a novel modified gellan gum.
2. Description of Related Art
Gums, also called hydrocolloids, are polysaccharides. Polysaccharides are polymers of simple sugar building blocks which have been in use since about 1900. Use of gums has increased throughout the century particularly in the past 40 years and today they are used in a wide variety of products and processes. Certain micro-organisms are capable of producing polysaccharides with properties differing from those of gums from more traditional sources. The best example of such microbially-produced polysaccharides is xanthan gum. More recently discovered examples are welan gum, rhamsan gum and gellan gum.
Gellan gum, first discovered in 1978, is produced by strains of the species Sphingomonas Elodea [formerly Pseudomonas Elodea], in particular strain ATCC 31461 [Kang, K. S. et al EP 12552 and U.S. Pat. Nos. 4,326,052; 4,326,053; 4,377,636 and 4,385,125]. Commercially this gum is produced as an extracellular product by aqueous cultivation of the micro-organisms in a medium containing appropriate carbon, organic and inorganic nitrogen and phosphate sources and suitable trace elements. The fermentation is carried out under sterile conditions with strict control of aeration, agitation, temperature and pH [Kang et al, Appl. Environ. Microbiol., 43, [1982], 1086]. When fermentation is complete, the produced viscous broth is pasteurised to kill viable cells prior to recovery of the gum. The gum can be recovered in several ways. Direct recovery from the broth yields the gum in its native or high acyl [HA] form. Recovery after deacylation by treatment with a base yields the gum in its low acyl [LA] form. Acyl groups present in the gum are found to influence its characteristics significantly.
The constituent sugars of gellan gum are glucose, glucuronic acid and rhamnose in the molar ratio of 2:1:1. These are linked together to give a primary structure comprising a linear tetrasaccharide repeat unit [O""Neill M. A., et al, Carbohydrate Res., 124, [1983], 123 and Jansson, P. E., et al., Carbohydrate Res., 124, [1983], 135]. In the native or high acyl [HA] form two acyl substituents, acetate and glycerate, are present. Both substituents are located on the same glucose residue and, on average, there is one glycerate per repeat unit and one acetate per every two repeat units. In the low acyl [LA] form, the acyl groups have been removed to produce a linear repeat unit substantially lacking such groups. Light scattering and intrinsic viscosity measurements indicate a molecular mass of approximately 5xc3x97105 daltons for [LA] gum [Grasdalen, H. et al., Carbohydrate Polymers, 7, [1987], 371]. X-ray diffraction analysis shows that gellan gum exists as a three-fold, left-handed, parallel double helix [Chandreskaran et al., Carbohydrate Res., 175, [1988], 1 181, [1988]23].
Low acyl [LA] gellan gums form gels when cooled in the presence of gel-promoting cations, preferably divalent cations, such as calcium and magnesium. The gels formed are firm and brittle. High acyl [HA] gellan gums do not require the presence of cations for gel formation and the gels formed have structural and Theological characteristics which are significantly affected by the acyl substituents. Thus the properties of [HA] gellan gels differ significantly from those of [LA] gellan gels. [HA] gels are typically soft and flexible and lack thermal hysteresis.
Typical gelation temperatures for [LA] gellan gums are in the range 30xc2x0 C. to 50xc2x0 C., depending upon the nature and concentration of the cations present. Typical gelation temperatures for [HA] gellan gums are in the region of 70xc2x0 C. The high gelation temperature of [HA] gellan gum can be advantageous in some applications such as fruit fillings where it can prevent flotation of the fruit. In other applications, however, such as ready-to-eat jellies and confectionery, the high gelation temperature can be a problem with regard to pre-gelation prior to depositing.
A wide range of gel textures can be produced through manipulation of blends of [HA] and [LA] gellan gum. However, it has been demonstrated that mixtures of [HA] and [LA] forms exhibit two separate conformational transitions at temperatures coincident with the individual components [Morris, E. R., et al., Carbohydrate Polymers, 30, [1996], 165-175]. No evidence for the formation of double helices having both [HA] and [LA] molecules has been found. This means that problems associated with the high gelation temperature of [HA] gellan gum still exist in blended systems.
It has been demonstrated that treatment conditions using strong bases such as potassium hydroxide during recovery influence both the composition and rheological properties of gellan gum [Baird, J. K., Talashek, T. A., and Chang, H., Proc. 6th International Conference on Gums and Stabilisers for the Food Industry, Wrexham, Clwyd, Wales. July 1991xe2x80x94Edited Phillips G. O., et al, published by IRL Press at OUP [1992], 479-487]. This suggests that control of acyl content by strong base treatment during the gum recovery process can lead to a diversity of textures. To date, however, this observation has not led to such control being realised on a commercial scale. Consequently, gellan gum remains available essentially in two forms only, i.e. [HA] and [LA].
Gellan gums have a wide variety of applications in food and non-food manufacture and the provision of a range of forms in addition to the basic [HA] and [LA] forms, i.e. a range of intermediate forms, other than blends, is desirable. Such new forms of gellan gums are potentially useful in the current search for suitable alternatives to gelatin.
According to the present invention there is provided a gellan gum composition which comprises a structure having linear tetrasaccharide repeat units of glucose residues to some of which residues are attached acetate and/or glycerate substituent groups wherein the ratio of acetate substituent groups to glycerate substituent groups is at least 1.
It should be noticed that the Baird at al. reference mentioned hereabove does not disclose gellan gum compositions that show acetate/glycerate ratios higher than 1.
Preferably, the ratio of acetate substituant to glycerate substituent is higher than 1.1.
According to the present invention, there is further provided a process for the modification of a gellan gum to alter the acyl content thereof which comprises a step wherein the gum is treated with a weak base, in an amount and under such conditions that the gel-forming properties and rheological behaviour of the gum are modified to an appreciable extent.
When processed by appropriate weak base treatment, the gellan gum composition may comprise a structure having linear tetrasaccharide repeat units of glucose residues to some of which residues are attached acetate and/or glycerate substituent groups wherein the total of attached acetate and glycerate substituent groups per linear tetrasaccharide repeat unit is at least 1 and the ratio of acetate substituents to glycerate substituents is at least 1.1 or, wherein the total acyl substituent groups per linear repeat unit is greater than 0.4 and less than 1 and the ratio of acetate substituent groups to glycerate substituent groups is at least 2.
Further, according to the present invention we provide a process for the production of a gellan gum-containing product which comprises the steps of:
a) modifying the gellan gum to alter the acyl content thereof by treating the gum with a weak base in an amount and under such conditions that the gel-forming properties and rheological behavior of the gum are modified to an appreciable extent; and later
b) causing the modified gum to gel during formation of the gellan gum-containing product.
Although, the prior art does not disclose gellan gum compositions as herein, a strong base treatment of native gellan gum is already known and can, under certain circumstances yield modified gellan gum as per this present invention.